Marker navigation system for detecting and displaying the location of marker means

ABSTRACT

A marker navigation system for detecting and displaying locations of at least two reference devices, each reference device attachable to a corresponding object includes a detection device for detecting signals emitted from the at least two reference devices, a display device for displaying the locations of the at least two reference devices and/or the objects attached to the at least two reference devices based in accordance with display signals; and a data processing device. The data processing device is configured to a) calculate locations of the at least two reference devices based on the detected signals; b) determine changes in the locations of the at least two reference devices based on the calculated locations; c) check, based on the determined changes, whether the changes in the locations of the at least two reference devices can be described by at least one checking transformation that, to a predetermined extent, transforms the locations of the at least two reference devices before the change into the locations of the at least two reference devices after the change; and d) determine the display signals for displaying the locations of the at least two reference devices and/or of objects attached to the at least two reference devices as a function of the checking result.

RELATED APPLICATION DATA

This application claims priority of U.S. Provisional Application No.60/862,070 filed on Oct. 19, 2006, which is incorporated herein byreference in its entirety.

FIELD OF THE INVENTION

The present invention relates to a marker navigation system fordetecting and displaying the location of marker means. The marker meanscan be attached to body structures such as, for example, bones orartificial limbs and/or to instruments, such as medical instruments.

BACKGROUND OF THE INVENTION

Conventional marker navigation systems, in particular so-calledimage-guided surgery (IGS) systems, generally detect and track thelocations of one or more reference frames that, for example, areconnected to a body structure (e.g., a bone) or to an instrument. Adisplay of a marker navigation system typically shows only a particularregion or section (e.g., a targeted region) of the operating area.

One problem with conventional marker navigation systems is that if achange occurs in the targeted region, it may be unclear whether thechange was due to camera movement or to movement of the observed markermeans (which for example may be connected to a body structure or aninstrument).

Another problem is that large relative movements of the marker means canoccur such that the objects connected to the respective marker means(e.g., the body structure or instrument), which are typicallycalibrated, are no longer displayed on the display (e.g., they havemoved out of a display region and are “off the display”). The latter maybe due to the fact that while a camera system, for example, detects ageneral region (e.g., the patients upper body), the surgeon may be onlyinterested in a sub-region (e.g., a portion of the patient's chest) ofthe general region, and only this sub-region is displayed of thedisplay. Thus, if the displayed objects of interest leave the sub-region(e.g., they move outside the display area), the surgeon may prefer thatthe display area be re-adjusted such that the objects of interest areagain provided on the display.

SUMMARY OF THE INVENTION

The present invention provides a device and method that optimizes thedisplay of marker means and/or objects connected thereto such that it isless frequently necessary to re-adjust a display area due to movement ofthe marker means and/or object. Further, absolute movements of a numberof objects (fragments) provided with marker means can be detected andoptimally displayed in a reference frame (e.g., a camera system).

Compensating for a drift in displayed objects advantageously enables themarker means and the objects connected thereto (e.g., bone fragments) tobe more informatively displayed in a reference frame (e.g., a camerareference frame). In particular, this enables independent tracking of anumber of fragments.

If, for example in accordance with a first scenario, a fragment A movesupwards, then the display shows that the fragment A is rising. If, inaccordance with a second scenario, a fragment B moves upwards, then thedisplay shows that the fragment B is rising. This has not been possiblein accordance with the prior art. The reference frame was bound to oneof the fragments A or B, e.g., fragment A was the display referenceframe and remained constant or stationary in said selectedrepresentation.

The first and second scenarios then differ in that in the first scenariothe fragment B drops and in the second scenario the fragment B rises.If, for example, a virtual center of mass of the fragments A and B istaken into account, even this does not enable the movement of thefragments to be displayed in a reference frame that, for example, liesin the operating theater or in the camera reference frame (lying in theoperating theater).

The present invention enables the absolute movement of a number offragments to be detected and displayed in a reference frame that lies inthe camera reference frame and/or in the space in which the user issituated. Compensating for drift helps in achieving this advantage, butalso represents a substantial advantage in its own right.

Marker means (e.g., a reference device such as a reference array or thelike) can be detected by means of a detection means (e.g., a camera orultrasound detector). The marker means typically comprise two to fourmarkers that are arranged in a fixed and predetermined location relativeto each other and may be mechanically connected. The markers can bepassive or active markers, wherein passive markers reflect signals(e.g., waves and/or radiation) emitted in their direction, and theactive markers are themselves the origin of the signals (e.g., radiationand/or waves). The signals emitted by the (active or passive) markers,which, for example, can be wave signals or radiation signals, can bedetected by a detection device (e.g., the camera). In order to establisha position of the marker means relative to the detection means, themarker means may be moved to provide the detection means with variousviews of the marker means. On this basis, the location of the markermeans relative to the detection means can be determined in a known way,in particular in a spatial reference frame. Reference is made in thisrespect to DE 196 39 615 A1 and corresponding U.S. Pat. No. 6,351,659,both of which are hereby incorporated by reference in their entirety.

The location of the marker means can be determined by the position ofthe marker means in a predetermined reference frame. The reference framecan be a reference frame in which the detection means lies. The locationof the marker means can be determined by the positions of the markers,in particular the center points of the markers in the reference frame.The positions, for example, can be described using Cartesian coordinatesor spherical coordinates. The location of one part (e.g., the detectionmeans or marker means) relative to another part (e.g., the marker means)can be described by spatial angles, distances, coordinates (in areference frame) and/or vectors, and is preferably calculated from thepositions describing the location, for example by means of a programrunning on a computer.

The term “relative location” as used herein or the expression “locationof a part A relative to a part B” thus comprises the concept of therelative positions between the two parts, in particular between themarker means and/or their markers or between a marker means (or itsmarkers) and the detection means. In particular, centers of gravity orcenter points of the parts may be selected as a punctiform referencepoint for establishing a position. If the position of one part is knownin a reference frame, then it is possible, on the basis of the relativelocation of two parts, to calculate the position of one of the two partsfrom the position of the other of the two parts.

The marker means can comprise at least two markers, and preferably threemarkers, and can of course also comprise more than three markers. Thedimensions of the markers and the locations of the markers relative toeach other may be known and available as prior known data of a dataprocessing means, or may have been determined by a calibration processduring execution of a data processing program. The shape of the markersis preferably also known.

The marker navigation system also can comprise a detection means thatdetects signals from the at least two markers, in particular from atleast two marker means (each comprising at least two markers). As statedabove, these signals may be emitted from the markers (either activelyemitted by the markers or reflected by the markers from another source).In the latter case, a signal transmitting source, for example anultrasound source or an infrared light source, can be provided thatemits signals (e.g., ultrasound waves or infrared light) towards thepassive markers (continuously or in pulses), wherein the passive markersreflect the signals. A data processing means, such as a computer, allowsthe location of the marker means relative to the detection means to becalculated, in particular the location of the marker means in areference frame in which the detection means lies, e.g., in a referenceframe that lies in an operating theater.

The data processing means can be designed to perform calculatingoperations, determining operations and checking operations. For example,the data processing means can calculate the locations of the markermeans on the basis of the detected signals emitted from the markermeans. Objects (e.g., a body structure or instruments) to which themarker means are attached are preferably calibrated. This means thatrelative locations at least between parts of the object and the markermeans attached to the object are known and/or stored in the dataprocessing means, such that display signals that describe the locationsof the objects can be determined on the basis of the locations of themarker means. The locations of the marker means can be calculatedrelative to the detection means, e.g., in a reference frame in which thedetection means lies. The locations can of course also be calculated inanother reference frame, e.g., in a reference frame in which one of themarker means lies and/or in which the patient lies.

The patient lies in the reference frame of the detection means (e.g.,the camera system) when the detection means and the patient are notmoving. This assumption can be made when calculating the location of theobjects. Additionally or alternatively, a marker means can be attachedto the patient, for example. This can be achieved by way of exampleusing a non-invasively attached marker means that is fixed to thepatient's body and can be detected by the detection means. One exampleof this is a so-called “headband”, in particular an “ENT headband”,which can be attached to various points on the patient's body, forexample on the extremities, by means of a band. A marker means can alsobe attached to the couch surface, in particular the table, on which thepatient is lying. The physician also can be selected as a referenceframe, for example, by providing the physician with a marker means(e.g., a headband). Lastly, the treatment room, e.g., the operatingroom, also can be selected as a reference frame. To this end, markermeans, for example, can be fixedly connected to the room, e.g., to thewall or walls.

The detection means may be designed to determine changes in thelocations of the marker means, and also changes in the reference frameof the detection means. The changes can be determined on the basis ofthe calculated locations and may be determined, for example, betweenparticular time intervals. This means, for example, that at particulartime intervals the detection means checks whether the calculatedlocations of the marker means have changed.

Instead of checking at particular intervals, a change in location may beconstantly checked and, once it is established that a change hasoccurred, then the extent of that change may be determined. The changesin the location of the marker means, for example, can be described byvectors in the reference frame of the detection means.

It also may be established whether determined changes in the location ofthe marker means can be attributed to a relative movement of the markermeans or whether it only appears to have been caused by a change in thelocation of the detection means and/or by a change in the location of anobject coupling the movement (called a coupling object below). Anexample of an object or coupling object is the patient or a couch onwhich a patient is lying. The coupling object can be characterized inthat the marker means have a stationary relationship with respect to thecoupling object if no external force acts on the marker means or theobjects connected to them.

It then may be determined whether the determined changes in thelocations of the marker means were caused by a movement of the detectionmeans or the coupling object. To this end, it may be determined whetherthe changes in the locations of (each of the marker means can or couldbe displayed by a (single) transformation, in particular atransformation matrix, which describes a change in the location of thedetection means or a coupling object. In particular, it may bedetermined whether the changes in location can be described bytranslating and rotating the reference frame in which the detectionmeans lies. In particular, it also may be determined whether the changesin location can be described by rotating and/or translating a referenceframe (ARF0) in which the marker means were lying before the change. Therotation and/or translation of the reference frame ARF0 can be likewisedescribed by a transformation, such as a transformation matrix. This canbe performed by means of numerical methods, with the aid of which acenter point of rotation can be determined. The aforementioned couplingobject can be characterized in that it lies in the reference frame ARF0.

In the case of a translational movement due to a change in the locationof the detection means or the coupling object, it will be seen that thedetermined changes in the location of the marker means and/or thereference frames assigned to them (ARF1, ARF2) can be described by thesame vector. A rotational movement of the coupling object or thedetection means can be detected by determining a center point of therotation and a rotational transformation (rotation matrix) that areidentical for all the marker means and/or assigned reference frames(ARF1, ARF2).

The data processing means can be configured to check the changes(changes in location), wherein the processing means checks whether thechanges in location (in particular all the changes in location) can orcould be described and/or explained by a (single) transformation. Ifthis is the case, a positive checking result may be assumed. Thistransformation is referred to here as the “checking transformation”. Thechecking transformation may cause a translation, a rotation, or acombination of a translation and a rotation.

A checking transformation can, but need not, be determined. It is alsosufficient to establish, using mathematical methods, whether the changesin location could be described or at least described to a predeterminedextent by a (single) transformation (checking transformation). If thechanges could be (at least substantially) described by a (single)checking transformation, then a positive checking result can be assumed.

A positive checking result also can be obtained without determining achecking transformation. This can be achieved, for example, as follows.Two of the marker means can be selected and designated as a first andsecond marker means. A relative transformation can be determined thattransforms a first marker reference frame, in which the first markermeans lies, into a second marker reference frame, in which the secondmarker means lies. It then can be observed and determined whether saidrelative transformation changes in the course of time, and if so, towhat extent. If, for example, said relative transformation remainsconstant, then a positive checking result may be assumed. This meansthat even if the location of the first marker means and the secondmarker means changes, for example in a camera reference frame (in whichthe camera is lying), this change can be attributed to a coupledmovement of the first marker means and the second marker means, sincethe relative transformation remains the same. The result of this is thatboth the change in the location of the first marker means and the changein the location of the second marker means in the camera reference framecould be described by the same checking transformation. A positivechecking result is thus obtained, without it being necessary tospecifically calculate the checking transformation.

It is sufficient to calculate the aforesaid relative transformation. Theextent of the change in the relative transformation, for example, can beestablished by determining a relative transformation at a first point intime and describing it using a matrix. This matrix can be multiplied bythe inverse of a second matrix, wherein the second matrix describes therelative transformation at a second point in time. If the result of themultiplication is a matrix that is similar to the unit matrix, then itmay be assumed that the relative transformation has not significantlychanged from the first point in time to the second point in time.

In order to obtain a measure of the similarity of the relativetransformations at the first and second points in time, the differencebetween the aforesaid matrix multiplication and a unit matrix can beformed. The elements of the resulting matrix then can be examined as totheir magnitude. The maximum value of the elements, for example, can bepicked out and compared with a threshold value. If it is smaller thanthe threshold value, then it is assumed that the change in the relativetransformation is minor, e.g., a positive checking result is obtained.Alternatively, the magnitudes of the elements of the differential matrixcan be added and compared with a threshold value. Other methods such asforming squares are likewise possible. The relative transformationpreferably describes at least one translation and/or rotation.

If there are more than two marker means, then it is possible todetermine a relative transformation between each of the two markermeans. Then, for example, it may be assumed that a positive checkingresult is obtained if no changes or only minor changes are seen in thecourse of time for a particular relative transformation, a number ofparticular relative transformations, or all the relative transformationsthat respectively link two reference frames of two marker means.

A checking transformation can be determined, for example, usingnumerical methods. The checking transformation also may be determinedwhen applying the checking transformation in the reference frame of thedetection means (CRF). This as has the result of the locations of (inparticular each of) the marker means relative to the detection systembefore the determined changes in location can be transferred into thenew locations of (in particular each on the marker means after thedetermined changes in location using the checking transformation. Inorder to discover a checking transformation, a first transformation, forexample, can be determined that describes the change in the location ofa first marker means. A second transformation then can be determinedthat describes the change in the location of a second marker means. Thefirst transformation, for example, then can be interpreted as a checkingtransformation and compared with the second transformation. If thechecking transformation (the first transformation) matches or at leastsubstantially matches the second transformation, then a positivechecking result can be determined.

The same applies if a checking transformation can be found that cantransfer the locations of the marker means in a common reference frameARF0 before the determined changes (changes in location) into thelocations after the determined changes (changes in location). In otherwords, if the locations of the marker means in the reference frame CRFor ARF0 are subjected to the checking transformation, then if a checkingresult is positive, then a compensation is made to the changes in thelocation of the marker means in the reference frame CRF or ARF0, e.g.,the locations of the marker means are returned back to their initiallocations (before the changes).

The above-mentioned checking transformation has been considered withregard to each of the marker means. In an alternative embodiment, apredetermined group can be selected from the detected marker means andonly a coupled movement of this group checked, for example, by onlydetermining relative transformations between marker means of this groupand examining how they change over time. Said group of marker means, forexample, can be a group of marker means that are fixedly connected tothe patient, while still other marker means, which may not be fixedlyconnected to the patient and with respect to which detecting a coupledmovement is not of interest, can be detected by the detection means.Thus, it may be checked whether the change in the location of aparticular group of detected marker means can or could be described by achecking transformation.

Applying the checking transformation has been described above such thatit is applied in a reference frame (CAF or ARF0) to the locations (whichare for example described by coordinates). Alternatively, this can ofcourse also be envisaged such that the checking transformation isapplied to the respective reference frame (CAF or ARF0), such that thecoordinates for the marker means (for example by rotating the referenceframe) are again the same as those which were obtained before thedetermined changes in location.

Determination of the checking transformation can include someuncertainties, or the determined changes in the locations of the markermeans may have been substantially, though not exclusively, caused by amovement of the detection means or the coupling object. This means thatthere also can be a certain degree of relative movement between themarker means.

As stated above, a positive checking result may be established if thechanges in location can be attributed to a movement of the detectionmeans and/or the coupling object. For the aforesaid reasons, a positivechecking result also can be established when (all of) the changes inlocation can be (at least) substantially described by a (single)checking transformation. In order to quantify the word “substantially”for data processing by the data processing means, a checkingtransformation is preferably determined that, once applied, results inthe smallest possible deviation between the locations of the markermeans before the determined changes and the locations of the markermeans after the determined changes. The marker means for which thelargest deviation occurred, for example, can be selected forquantification. A first deviation between the first location (thelocation before the determined change) and the second location (thelocation after the determined change and after the checkingtransformation for compensating for the change in location has beenapplied) can be quantified, for example, by the distance between thefirst and second location. A second deviation also can be determined asthe difference between the first location (the location before thedetermined change in location) and a third location (the location afterthe determined change in location and without a checking transformationbeing applied). The first deviation then can be related to the seconddeviation. If this relationship is smaller than a predeterminedpercentage, then the checking result may be regarded as being positive.The predetermined percentage, for example, can measure less than 50%,25%, 10%, 5% or 1%. If the criterion “less than a predeterminedpercentage” is fulfilled, then it is assumed that the changes in thelocations of each of the marker means can be described at least to apredetermined extent by the checking transformation.

As an alternative to or in addition to forming the aforesaidrelationship, the cited criterion “substantially” can also be quantifiedby absolute threshold values. If, for example, the first deviation issmaller than a pre-set threshold value, it can be assumed that thechecking result is positive, e.g., the change in location can besubstantially described by the checking transformation. Alternatively oradditionally, a positive checking result can be assumed when the seconddeviation is above a particular threshold value. In particular, a numberof first threshold values can be established for the first deviation anda number of second threshold values can be established for the seconddeviation in the aforesaid sense, wherein each first threshold value canbe assigned to a second threshold value (e.g., in the form of a table),and a positive checking result can be assumed when the first deviationfalls below the first threshold value and the second threshold value isexceeded by the second deviation.

Display signals that show the locations of the marker means and/or theobjects connected to the marker means relative to a display referenceframe can be determined as a function of the checking result. This meansthat if a checking result is positive, the detected changes in locationare not further processed and the present display signal transmitted tothe display means is retained.

Determining display signals as a function of the checking result doesnot exclude also determining the display signals as a function of other(optional) conditions. These conditions can include whether the objectsdisplayed before the change in location could also be seen after saidchange in location has been displayed on a display means (e.g., ascreen). Another optional condition, as a function of which the displaysignals can be determined, can also be represented by the speed of thechange in location. The speed can be calculated from the extent of thechange in location (i.e., the distance between the aforementioned firstlocation and third location) divided by the time in which the change inlocation occurred. It is for example possible to only display changes inlocations that occur at a speed which is above or below a particularspeed threshold value.

Alternatively or additionally, changes in location can be prevented frombeing displayed if the changes in location occur at a speed that isabove a first speed threshold value and/or below a second particularspeed threshold value. The aforesaid conditions can be used, forexample, such that only rapid changes in location are displayed,provided only such rapid changes are of interest to the surgeon.Alternatively, the rapid changes in location can be suppressed if, forexample, they represent a movement that is caused by vibrations and is adistraction to the surgeon.

Thus, changes in the locations may not be displayed by the displaymeans, in particular as a function of the checking result (andoptionally also as a function of other conditions). This can beimplemented by the data processing means by leaving the display signalsunchanged. Alternatively, the display area or the reference frame of thedisplay in which the marker means and/or the objects connected to themare displayed can be subjected to the determined checkingtransformation, such that a change is not seen on the display or onlythose changes are shown that are not compensated for by applying thechecking transformation. The result of the latter is that if there is anoverlap of a relative movement between the marker means and a movementof the detection means and/or the coupling object, (only) the relativemovement is shown, whereas the changes due to the movement of thedetection means and/or the coupling object are not shown.

Thus, if the checking result is positive, the usual processing of thedetection signals is changed. The usual representation may be retainedeven when a checking result is positive, but only as long aspredetermined objects (which for example have hitherto been displayed bythe display means) continue to remain visible on the display means.

It can occur, for example, that a drift is not completely compensated.The cause of this can be numerical inaccuracies or the aforesaidconditions, if compensations are made for only changes in location atspeeds which are within a predetermined range of speeds. Another causeof the predetermined objects leaving the display area can be absolutemovements of (all the) displayed objects in one direction. For theaforesaid scenarios, it thus can occur that at least one of thedisplayed objects moves out of the visible range, e.g., is onlypartially displayed. The displayed objects then can be re-aligned, inparticular centered, such that all the objects are completely displayedby the display means. To this end, the data processing means can bedesigned such that it sets a constant minimum distance from thedisplayed objects to the edge of the displayed area. In other words,re-centering can keep the minimum distance from each object to the edgeof the display area constant. The displayed objects can be manuallyre-aligned by the operator or also automatically re-aligned, where inthe latter case, the re-alignment may be indicated to the operator by awarning signal.

The data processing means also can be configured to determine thechecking transformation as follows. For each detected object, at leastfor each object displayed on the display means, a transformation can becalculated that is referred to here as a change transformation. Thechange transformation transforms the first location (before the change)into the third location (after the change) for each object. The changetransformation can be performed in the reference frame ARF0 in which thereference frames of the (displayed) objects lay before the change.

In a subsequent step, the change transformations for the individual(displayed) objects then can be compared with each other. If the changetransformations are the same, a positive checking result can beestablished. A positive checking result also can be established if thechange transformations are similar to a predetermined degree or extent.The latter means that when one of the determined change transformationsis applied to the first location of all the displayed objects, alocation can be respectively obtained that only deviates to a minorextent from the third location (after the change in location) for eachobject. A minor extent means that the relationship of the deviationrelative to the extent of the change in location is smaller than apredetermined percentage. The predetermined percentage measures forexample 25%, 10%, 5% or 1%.

The data processing means can determine the checking transformation fromthe change transformations determined for the respective displayedobjects, for example, by averaging the change transformations or byselecting one of the change transformations.

As already mentioned above, it is possible to calculate a speed of thechange. To this end, the distance between the third location (after thechange) and the first location (before the change) can be calculated foran object, and divided by the time in which said change in locationoccurs. If the speed is below a pre-set value, then if a checking resultis positive, a change in the locations is preferably not displayed. Ifthe speed is equal to or above the threshold amount, then the detectedchange in location is preferably displayed. The threshold value for thespeed is for example above 0.1 mm/s, 1 mm/s, 3 mm/s or 1 cm/s and/orpreferably below 10 cm/s, 3 cm/s, 1 cm/s, 1 mm/s. In this way, it ispossible to eliminate a slow drift of the displayed objects.

The change in location may not displayed when the speed is above thethreshold value, even if otherwise (i.e., when the change in location isdisplayed), at least one of the displayed objects would at leastpartially leave the displayed area.

BRIEF DESCRIPTION OF THE DRAWINGS

The forgoing and other features of the invention are hereinafterdiscussed with reference to the drawing.

FIG. 1 shows an exemplary marker navigation system in accordance withthe invention.

FIG. 2 is a block diagram of an exemplary computer system that may beused to carry out one or more of the methods described herein.

DETAILED DESCRIPTION

FIG. 1 shows an exemplary marker navigation system in accordance withthe invention, wherein reference stars 3 a and 3 b are attached boneparts 20 a and 20 b, respectively, and wherein the bone parts can bemoved relative to each other. The reference stars 3 a and 3 b are eachan example of a marker means and comprise three marker spheres arrangedin characteristic locations relative to each other.

A camera 1 is also provided that serves as a detection means. The camera1, which is connected to a data processing means 2 (e.g., a computer)via a data signal line, comprises detection elements 1 a and 1 b. Acamera reference frame (CRF) is assigned to the camera 1, a referenceframe ARF1 (array reference frame) is assigned to the reference star 3a, and a reference frame ARF2 is assigned to the reference star 3 b. Thecamera 1 lies in the reference frame CRF, the reference star 3 a lies inthe reference frame ARF1, and the reference star 3 b lies in thereference frame ARF2. The two detection elements 1 a and 1 b, with theaid of the data processing means 2, are preferably provided to enable(in a manner similar to human spatial perception) a calculation of thelocation of the reference stars 3 a and 3 b relative to the detectionmeans 1 based on a detection signal.

When using the marker navigation system of FIG. 1, the followingsituation can then occur. A location of the reference stars 3 a and 3 bis not changed, but a location of the camera 1 is changed. If the changein the camera location is not taken into account, the display of thebone parts 20 a′ and 20 b′ on a display 30 changes, even though the boneparts 20 a and 20 b have not actually moved. Another, comparablesituation, for example, is when the patient is moved while lying on asupport base (the coupling object). These movements can mean that thebone parts 20 a′ and 20 b′ disappear from the displayed area of thedisplay 30. The latter also may be undesirable.

To compensate for the above, a movement of the camera 1 and/or acoupling object (e.g., the patient or couch) is deduced when suchdetected movement can be explained by a movement of the camera 1 and/orthe coupling object. The reference frame ARF0 shown in FIG. 1 isconnected to the coupling object.

If, for example, a shift in each of the reference stars 3 a and 3 b bythe same vector A is detected in FIG. 1, this can be explained by ashift in the patient or for example the support surface (on which thepatient is lying). In this case, it is possible to prevent such shiftfrom being displayed. This can be achieved, for example, by not changingthe display signals transmitted from the data processing means 2 to thedisplay 30, despite the detected change. Alternatively, the displayreference frame CRF (i.e., the camera reference frame) can likewise beshifted by the vector A, such that there is no change in the position ofthe bones 20 a′ and 20 b′ in the reference frame CRF and thus also nochange to be seen on the display 30.

Using numerical methods, it can be ascertained from the detected changein the reference stars 3 a and 3 b whether said change can be describedby a transformation matrix, wherein the transformation matrix can bedescribed by a translation or rotation of the reference frame CRF or areference frame ARF0. The reference frames of the reference stars (herethe reference frames ARF1 and ARF2 before the change) lie in thereference frame ARF0.

A check as to whether the changes in the location of two marker meanscould be described by a (single) checking transformation, for example,can be made as follows. In a specific implementation, the positions oftwo marker arrays A and B are detected in camera coordinates, whereinthe positions are expressed by coordinate transformations M_(A) andM_(B) from the reference frame of the camera to the respective referenceframe A or B. The coordinate transformations can be substantiallyrepresented by 4×4 matrices, and generally contain a degree of rotationand a degree of translation.

At an initial point in time t₀, the coordinate transformations M_(A)(t₀)and M_(B)(t₀) are detected and stored. In addition, an initial relativecoordinate transformation M_(BA),_(last) is stored. At consecutivepoints in time t_(i), the coordinate transformations M_(A)(t_(i)) andM_(B)(t_(i)) are detected again, from which the relative coordinatetransformation M_(BA)(t_(i)) is ascertained.

Then an initial check is performed to determine whether there is acoupled movement of the arrays A and B, by forming a “differentialmatrix” from the ascertained relative coordinate transformations:

ΔM _(BA)(t _(i))=M _(BA)(t _(i))*(M _(BA),_(last))⁻¹−1₄(the latter isthe four-dimensional unit matrix)

The criterion for a couple movement and therefore for a positivechecking result is that the difference between consecutive relativepositions is very small. Mathematically, for example, this can beinterpreted such that the maximum norm of the calculated differentialmatrix must be smaller than a pre-set absolute threshold value:

∥ΔM_(BA)(t_(i))∥max 21 threshold value

That is, the element of the matrix having the highest value must besmaller than the threshold value. Next, the value of M_(BA)(t_(i)) isassigned to the matrix M_(BA),_(last), such that at the point in timet_(i+1), it is again possible to refer to the previous relativeposition.

If a coupled movement has been identified (i.e., if the checking resultis positive), then such movement is suppressed on the display 30 of themarker means. As a result, none of the marker means and/or none of thebodies connected to the marker means are moved on the display 30.

If there is no coupled movement, then each of the marker means A and Bis moved on the display 30 in the way said movement is represented inthe camera system. To this end, the relative shift in the position isindividually calculated for each marker array, in a similar way toabove, by comparing it with the position from the previous step in time.If there previously was a coupled movement, then the position was notstored again so as to enable only the non-coupled movement to bedisplayed.

Moving now to FIG. 2 there is shown a block diagram of an exemplarycomputer 2 that may be used to implement one or more of the methodsdescribed herein. The computer 2 may include a display 30 for viewingsystem information, and a keyboard 32 and pointing device 34 for dataentry, screen navigation, etc. A computer mouse or other device thatpoints to or otherwise identifies a location, action, etc., e.g., by apoint and click method or some other method, are examples of a pointingdevice 34. Alternatively, a touch screen (not shown) may be used inplace of the keyboard 32 and pointing device 34. The display 30,keyboard 32 and mouse 34 communicate with a processor via aninput/output device 36, such as a video card and/or serial port (e.g., aUSB port or the like).

A processor 38, such as an AMD Athlon 64® processor or an Intel PentiumIV® processor, combined with a memory 40 execute programs to performvarious functions, such as data entry, numerical calculations, screendisplay, system setup, etc. The memory 40 may comprise several devices,including volatile and non-volatile memory components. Accordingly, thememory 40 may include, for example, random access memory (RAM),read-only memory (ROM), hard disks, floppy disks, optical disks (e.g.,CDs and DVDs), tapes, flash devices and/or other memory components, plusassociated drives, players and/or readers for the memory devices. Theprocessor 38 and the memory 40 are coupled using a local interface (notshown). The local interface may be, for example, a data bus withaccompanying control bus, a network, or other subsystem.

The memory may form part of a storage medium for storing information,such as application data, screen information, programs, etc., part ofwhich may be in the form of a database. The storage medium may be a harddrive, for example, or any other storage means that can retain data,including other magnetic and/or optical storage devices. A networkinterface card (NIC) 42 allows the computer 2 to communicate with otherdevices.

A person having ordinary skill in the art of computer programming andapplications of programming for computer systems would be able in viewof the description provided herein to program a computer 2 to operateand to carry out the functions described herein. Accordingly, details asto the specific programming code have been omitted for the sake ofbrevity. Also, while software in the memory 40 or in some other memoryof the computer and/or server may be used to allow the system to carryout the functions and features described herein in accordance with thepreferred embodiment of the invention, such functions and features alsocould be carried out via dedicated hardware, firmware, software, orcombinations thereof, without departing from the scope of the invention.

Computer program elements of the invention may be embodied in hardwareand/or in software (including firmware, resident software, micro-code,etc.). The invention may take the form of a computer program product,which can be embodied by a computer-usable or computer-readable storagemedium having computer-usable or computer-readable program instructions,“code” or a “computer program” embodied in the medium for use by or inconnection with the instruction execution system. In the context of thisdocument, a computer-usable or computer-readable medium may be anymedium that can contain, store, communicate, propagate, or transport theprogram for use by or in connection with the instruction executionsystem, apparatus, or device. The computer-usable or computer-readablemedium may be, for example but not limited to, an electronic, magnetic,optical, electromagnetic, infrared, or semiconductor system, apparatus,device, or propagation medium such as the Internet. Note that thecomputer-usable or computer-readable medium could even be paper oranother suitable medium upon which the program is printed, as theprogram can be electronically captured, via, for instance, opticalscanning of the paper or other medium, then compiled, interpreted, orotherwise processed in a suitable manner. The computer program productand any software and hardware described herein form the various meansfor carrying out the functions of the invention in the exampleembodiments.

Although the invention has been shown and described with respect to acertain preferred embodiment or embodiments, it is obvious thatequivalent alterations and modifications will occur to others skilled inthe art upon the reading and understanding of this specification and theannexed drawings. In particular regard to the various functionsperformed by the above described elements (components, assemblies,devices, compositions, etc.), the terms (including a reference to a“means”) used to describe such elements are intended to correspond,unless otherwise indicated, to any element which performs the specifiedfunction of the described element (i.e., that is functionallyequivalent), even though not structurally equivalent to the disclosedstructure which performs the function in the herein illustratedexemplary embodiment or embodiments of the invention. In addition, whilea particular feature of the invention may have been described above withrespect to only one or more of several illustrated embodiments, suchfeature may be combined with one or more other features of the otherembodiments, as may be desired and advantageous for any given orparticular application.

1. A marker navigation system for detecting and displaying locations ofat least two reference devices, each reference device attachable to acorresponding object, comprising: a detection device for detectingsignals emitted from the at least two reference devices; a displaydevice for displaying, based in accordance with display signals, thelocations of the at least two reference devices and/or the objectsattached to the at least two reference devices; and a data processingdevice configured to a) calculate the locations of the at least tworeference devices based on the detected signals; b) determine changes inthe locations of the at least two reference devices based on thecalculated locations; c) check, based on the determined changes, whetherthe changes in the locations of the at least two reference devices canbe described by at least one checking transformation that, to apredetermined extent, transforms the locations of the at least tworeference devices before the change into the locations of the at leasttwo reference devices after the change; and d) determine the displaysignals for displaying the locations of the at least two referencedevices and/or of objects attached to the at least two reference devicesas a function of the checking result.
 2. The device according to claim1, wherein the object is a body structure and/or a medical instrument.3. The marker navigation system according to claim 1, wherein the dataprocessing device is configured to set a positive checking result whenat least one relative transformation does not change or changes lessthan a predetermined extent over time, wherein said at least onerelative transformation transforms a transformation from a firstreference device reference frame in which a first reference device ofthe at least two reference devices lies into a second reference devicereference frame in which a second reference device of the at least tworeference devices lies.
 4. The marker navigation system according toclaim 3, wherein if the checking result is positive, the data processingdevice is configured to not include or only partially include thedetermined changes in the display signals.
 5. The marker navigationsystem according to claim 3, wherein if the checking result is positive,the data processing device is configured to retain the display signalsthat do not reflect the determined changes.
 6. The marker navigationsystem according to claim 3, wherein if the checking result is positive,the data processing device is configured to calculate a speed of thechanges and, if the calculated speed is less than or greater than apredetermined speed, then the display signals are unchanged or thedisplay signals are determined such that the determined changes are notdisplayed or are only partially displayed.
 7. The marker navigationsystem according to claim 6, wherein if the calculated speed is greaterthan or equal to the predetermined speed, the data processing device isconfigured to not display or partially display the determined changes,and if the locations that have changed in accordance with the determinedchanges are displayed, all the previously displayed locations are notdisplayed on the display device.
 8. The marker navigation systemaccording to claim 1, wherein if a checking result is positive, the dataprocessing device is configured such that the locations of the at leasttwo reference devices after the change in location are subjected to atransformation that at least partially reverses the change in thelocation, and the display signals are calculated on the basis of thetransformed locations.
 9. The marker navigation system according toclaim 8, wherein the transformation corresponds to or is calculated fromthe at least one checking transformation.
 10. The marker navigationsystem according to claim 1, wherein if the checking result is positive,the data processing device is configured to leave the display signalsunchanged, such that the display device does not display any change inthe locations or the determined changes are only partially displayedand/or the changed and determined locations are subjected to thechecking transformation before determining the display signals, suchthat the changes are at least partially reversed.
 11. The markernavigation system according to claim 1, wherein the data processingdevice is configured to determine the at least one checkingtransformation based on the assumption that: a) the determined changeshave been caused by rotating and/or translating the detection means;and/or b) the determined changes have been caused by rotating and/ortranslating an object that is stationary relative to the at least tworeference devices.
 12. The marker navigation system according to claim1, wherein the data processing device is configured to: determine the atleast one checking transformation by calculating for each of the atleast two reference devices a change transformation that describes thechange in the location from before the change to after the change;compare the calculated change transformations with each other; andestablish a positive checking result if all the change transformationsare within a predetermined range of one another.
 13. The markernavigation system according to claim 12, wherein the data processingdevice is configured to determine the at least one checkingtransformation from the change transformations.
 14. A method fordetecting and displaying a location of at least two reference devicesvia a marker navigation system, wherein each reference device isattachable to a corresponding object, comprising: detecting signalsemitted from the at least two reference devices; calculating locationsof the at least two reference device based on the detected signals;determining changes in the locations of the at least two referencedevices based on the calculated locations; checking, based on thedetermined changes, whether the changes in the location of each of theat least two reference devices can be described by a checkingtransformation that is the same for all the changes and that at least toa predetermined extent transforms the locations of the at least twomarkers before the change into the locations of the at least two markersafter the change; determining, as a function of the checking result,display signals that display the locations of the at least two markersand/or objects attached to the at least two markers; displaying thelocations in accordance with the display signals.